It is known to remedy this disadvantage by accumulating the washing water, during the water and air washing period, in a storage capacity disposed above the discharge chute, said chute being provided with a device such as a flap, a grid, a seeve, . . . which allows the total or partial closing of the chute during the washing period.
Such known devices have the disadvantage of a large and useless height outside the washing period; moreover, they necessitate a certain period for the decantation of the filtration material and also of the extracted impurities, and therefore a water flow rate which is all the more important that the distance to be covered by the washing water up to the discharge chute is great.
Other devices have been proposed, consisting in vertical or slanting walls placed in the vicinity around the washing water discharge chute, said walls being designed for deflecting the air flow entraining the material grains, thereby creating a favourable zone for the decantation of said grains. Moreover, on the other hand, these known devices include generally a horizontal grid disposed above the filtration material and therefore likely to be obstructed during the wahing period by the extracted impurities and the filtration material itself, thereby causing a reduction of the system efficiency.
On the other hand, said known devices are applicable only to the collection of the washing water and are not usable for the collection of the water treated in the granular material bed filters used for the biological clarification on fixed cultivations and in which the water and air are injected from bottom to top during the treatment phase as well as during the material washing phase: in said filters indeed, the known devices designed for a periodical usage are not sufficient for avoiding turbulence phenomena due to the permanent blowing of air in the filter. Moreover, when used continuously, they get rapidly clogged.